Zircon textures and composition:Refractory recorders of magmatic volatile evolution?
Zircon textures and composition have been used to infer magmatic processes including closed-system fractional crystallization, magma mixing or replenishment, and country-rock assimilation. Here, we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evo...
| Published in: | Contributions to Mineralogy and Petrology |
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| Main Authors: | , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2013
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| Subjects: | |
| Online Access: | https://researchers.mq.edu.au/en/publications/03c3c32f-ebb6-4c54-82e9-d97bb6d7356e https://doi.org/10.1007/s00410-012-0791-z http://www.scopus.com/inward/record.url?scp=84871936908&partnerID=8YFLogxK |
| Summary: | Zircon textures and composition have been used to infer magmatic processes including closed-system fractional crystallization, magma mixing or replenishment, and country-rock assimilation. Here, we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution. We present field, whole-rock chemical, textural, mineral chemical, and U-Pb age data from evolved, fine-to-coarse-grained granite intrusions on Melville Peninsula, Nunavut, Canada. Zircon forms two main populations in these granites, Type-1 and Type-2 zircon. Type-1 zircon is present in all samples, but predominant in fine-grained granite. Crystals are euhedral and inclusion-rich and show periodic, fine-scale oscillatory zoning, comparatively low concentrations of U (< 2,200 ppm) and Hf (< 1. 6 wt%), high Zr/Hf (~40-62), and pervasive alteration. Type-2 zircon is predominant in coarse-grained granite. Crystals form overgrowths on Type-1 zircon and individual crystals. They are subhedral and inclusion-poor and show weak, irregular, large-scale oscillatory zoning, high U (up to ~7,250 ppm) and Hf (1. 5-2. 0 wt%), low Zr/Hf (~37-44), and only local alteration. Compatible trace-element concentrations and Zr/Hf change sharply across the boundary of Type-1 to Type-2 zircon; 207 Pb/ 206 Pb ages preclude a significant hiatus between crystallization of the two types. We argue against magmatic versus hydrothermal crystallization, country-rock assimilation, or magma mixing as causes for the crystallization of Type-1 and Type-2 zircon. We propose instead that Type-1 zircon formed from volatile-undersaturated magmas and that Type-2 zircon formed from volatile-saturated magmas. Magmas fractionated by volatile-driven filter pressing into crystal-rich mush and crystal-poor magma. Crystal-rich mush with abundant Type-1 zircon crystallized to fine-grained granite. Volatile-rich magma crystallized to Type-2 zircon and coarse-grained granite. While Type-1 zircon was pervasively altered by exsolving magmatic volatiles, ... |
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